The Effect of Sandblasting and Coating of Zirconia by Nano Composites on Bond Strength of Zirconia to Resin Cements

Statement of the Problem: Despite yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) high strength in dental restoration application, Zr- the framework has a low tendency to react chemically with cement which is the main reason of these restoration failures. Purpose: The aim of this in vitro study was to evaluate the effect of Y-TZP coating by nanocomposite of silica and aluminosilicate according to the sol-gel dip-coating technique on the bond strength of resin cement to Y-TZP. Materials and Method: In this experimental study, Y-TZP blocks (10×10×3mm3) were prepared and sintered and assigned into 4 groups (n=10) for coating including control group without any further surface treatment, sandblasted using 110μm alumina particles under 2.5 bar and tip distance of 10 mm, silica sol dip coating+calcination, aluminosilicate sol dip coating+ calcinations. To confirm chemical bonds of sol-gel covers, Fourier transforms infrared spectroscopy (FT-IR) technique was used. The surface of the sample was investigated by scanning electron microscopy (SEM), energy-dispersive spectroscopy detector (EDS) and x-ray diffraction (XRD) methods. Micro-shear bond strengths (µSBS) of zirconia-cement specimens were evaluated. Data were analyzed with a one-way ANOVA test in SPSS version 11.5 software with a confidence interval of 95%. Results: µSBS of sandblasting, nano-silica, and nano-aluminosilicate specimens were significantly higher than control. µSBS of nano-silica was higher than other groups but no significant difference was observed in µSBS of sandblasting nano-silica, and nano-aluminosilicate groups (p> 0.05). Conclusion: Covering the zirconia surface with non-invasive nano-silica and nano-aluminosilicate using the sol-gel technique leads to improved cement bond strength.


Introduction
Surface preparation of zirconia-based dental restoration for resin bonding is still being considered as a major challenge. Unlike feldspathic porcelain and glassceramics, zirconia cannot be etched by hydrofluoric acid because its structure is completely made of crystalline phase and no glassy matrix exists. Therefore, a method other than acid etching must be used to provide microstructures for creating micromechanical adhesion or chemical bonding to improve the zirconia-resin bonding. So far, many researchers have examined different methods for creating zirconia surface roughness and improving zirconia-resin bonding [1][2][3][4][5]. Various methods were introduced including; application of phosphate monomers (MDP), special primers, formulation of new silane, applying low fusing porcelain, slurry zirconia  [27]. In this study, silica and aluminosilicate have been used through the dip coating sol-gel method for zirconia coating to improve the bond strength of zirconia to resin cement. The null hypothesis tested was that the coating of Y-TZP surface functionalization by different coatings has no effect on the resin-zirconia bond strength.

Materials and Method
Specimen preparation  Table 1. To provide a clean surface before coating, the blocks were washed under running distilled water, placed in an acetone/ethanol ultrasonic bath (Elmasonic S 300 H) for 20 minutes, and then air-dried at 70°C in Chamber Furnace -ELF11/6 [27].

Sol preparation
The silica sol was prepared as described previously [28]. were expressed in our previous study [27].

Coating characterization
FT-IR, XRD, SEM and EDS were used to characterize the coating and the nature of the bonding between the coating and zirconia. The methods of these tests which we used in this study described in our previous article completely [27].

Micro-shear-bond strength testing
To evaluate the efficacy of bonding between Y-TZP and

Results
The results of the analysis of the FT-IR, XRD, SEM, and EDS tests are explained in our previous study [27].
The results of the µSBS test for each study group are shown in Figure 2. Statistical analysis revealed that there were significant differences between the study groups.  groups was studied using a stereo microscope. The results showed that the fracture mode in all four groups was adhesive type.

Discussion
The use of zirconia as a core in dental crowns improves fixed prosthesis due to high strength, tissue compatibility, and beauty. However, one of the main disadvantages of the Zirconia framework is the low tendency to react chemically with cement and thereby its weak bond. Thus, various techniques and materials have been used for improving the bond between zirconia and resin cement [1][2][3][4][5]. The aim of the present study was to investigate the bond strength between zirconia and resin ce- This reaction and formation of a new layer on the zirconia surface were confirmed by SEM, EDS, FTIR and XRD analysis in our previous study [27].
The self-adhesive luting cement (Clearfil SA, Kurary,Japan) used in this study is an MDP-based silane containing resin cement. Previous studies showed that these characteristics have improved the bonding between resin cement and neutral surfaces like zirconia and alumina core surface or silica-based surface due to silanization [31][32][33]. Therefore, this cement was used for its high adhesive strength to Y-TZP ceramics.  [23][24][25]. Some authors recommended sandblasting with particle size below 50 µm, applying to sandblast at low pressure (1 to 2 bars) in order to minimize zirconia surface microcrack [26].
According to Chen C et al. [14], cement bond strength to zirconia in samples coated with nano-silica by sol-gel method was significantly higher than sandblasted samples. However, Lung C et al. [17] found that the bond strength of composite resin to zirconia prepared by sandblasting was higher than the samples, which were coated with silica by using sol-gel method.
The differences observed, can be attributed to the method used by Lung C et al. [17] [18] found no increase in resin bond strength to zirconia coated with a layer of nano-alumina particles. The reactive magnetron sputtering method had been used in their study. On the other hand, Jevincar P et al. [16] and Zhang et al. [20] found an increase in resin bond strength after nano alumina coating by immersing the blocks in the AlN suspension. In this study, aluminosilicate coating improved the µSBS but this strength is less than sandblast and silica-coated groups, this is probably due to silica particles that fill the gaps between alumina particles and inhibit the resin tag formation, which is important in bond strength.

Conclusion
According to the results of the present study, the use of nano-silica and aluminosilicate coatings on zirconia can increase the µSBS of resin cement to zirconia. Therefore, methods other than a mechanical procedure like thribochemical coating technique, which may have long destructive effects on the zirconia-resin cement bonding, could be used.